The present invention relates to a device of improved type for the separation of airborne particles such as an aerosol, into grain size classes. Such a device is an instrument which permits the deposition onto a filter of dust particles on which it is then possible to perform a series of chemical and physico-chemical analyses in dependence on the dimensions thereof, for the purpose of determining the risk from inhalation of dust present in a predetermined environment. The said device is utilised in the testing of industrial and environmental health in medical physics and in powder technology generally.
Devices currently in commercial use consist schematically of an L-shaped channel of rectangular outline through which filtered air is passed. This channel thus has two parts the first of which is defined by a first nozzle which ejects filtered air into the second part which is delimited above by a body and below by a support plate on which a filter is positioned. Within the first nozzle there is located a second nozzle which ejects the particle-bearing air or aerosol into a zone of the first part of the channel itself. In this way those particles of dust which have the same dimensions tend to be disposed within the same band of fluid flow which passes out downstream of the said curvature; it is thus possible to think of the fluid as a plurality of bands each of which includes the particles of a single grain size class. The successive bands are deposited on the filter starting from the band with the particles of greater diameter. Experimentally it is noted that the particles of smaller diameter are not clearly separated from one another and that these are deposited together in the portions of the filter spaced furthest from the curvature.
The devices described above have various disadvantages.
In particular, they have a low sampling capacity for the purpose of being utilised in testing environmental health. Moreover, if a diagram is constructed on the abscissa of which is plotted the distance of deposition of the particles from the curvature of the channel, and on the ordinate of which is plotted the aerodynamic diameter of the particles, there is obtained a curve which is linear in the first part relating to the particles of greater diameter, and which is nonlinear and very steep in the second part, relating to the particles of smaller diameter, which may be, however, those of greatest interest in the testing of environmental hygiene, that is to say for determining the toxicity of certain inhalations. This non-linearity of the curve of the diagram for the particles of smaller diameter is caused by the fact that the devices currently used do not succeed in separating completely the particles of such grain size classes from one another, with the consequence that, in a certain range of diametral dimensions; the particles are deposited on the same portion of the filter. In conclusion, the orthogonal projection of the aerosol stream onto the plane of the filter permits a distinct separation only between the bands with particles having a larger diameter.